This invention relates to the art of magnetic sensing devices and, more particularly, to an improved magnetic sensor for use in a system for measuring the position, velocity and/or direction of a moving object having alternating zones of different magnetic conductivity in the direction of movement. This invention also relates to my prior co-pending application, Ser. No. 437,657 filed Nov. 17, 1989, now U.S. Pat. No. 4,992,733, with respect to which the present application is a continuation-in-part.
It is, of course, well known to provide a transducer or motion sensor comprised of a permanent magnet having a pole face facing an object whose motion is to be detected and which pole face is provided with a magnetic field sensitive sensor element coaxial with the axis of the pole face. Such a motion sensing device is adapted to be positioned with the pole face and sensor element facing and spaced by an air gap from an object such as an axially reciprocable rod or a rotatable toothed gear formed of a magnetically conducting material. The rod may, for example, be provided with circumferentially extending grooves or threads, providing alternating lands and grooves, and the teeth of the gear are of course circumferentially spaced apart, whereby the rod lands and grooves, and the gear teeth and spaces therebetween, provide alternating zones of different magnetic conductivity in the direction of movement of the object. The lines of flux of the magnetic field of the permanent magnet pass through the alternating zones as the object moves relative to the magnetic sensor, causing flux changes and inducing a current in the magnetic sensor which is proportional to the position of the moving object. The output of the magnetic sensor is applied to a suitable auxiliary electronic device to provide some desired function or display. It will be appreciated of course that the magnetic field of the permanent magnet is uniform with respect to a magnetic sensor centered on the pole face thereof so as to be coaxial with the axis of the pole face.
Often, it is desirable in connection with such a motion detecting device to provide for determining the direction of movement of the object. This can be achieved by simply providing two magnetic sensor elements on the pole face on opposite sides of the axis of the pole face and spaced apart in the direction of movement of the object by a distance proportional to the pitch of the rod lands and grooves or gear teeth. With such an arrangement, the flux changes at any given time resulting from the passage of the zones of different magnetic conductivity relative to each of the magnetic sensors will be different, thus producing different signals at the auxiliary electronic device capable of providing directional information with respect to the moving object. However, the simple mounting of two magnetic sensor devices on the pole face causes problems with respect to the signal output from the individual magnetic sensors. In this respect, the offset of the two magnetic sensors relative to the center of the magnet provides a magnetic field in the region of each of the magnetic sensors that is not uniform. In this respect, the magnetic flux on the side of each sensor which is nearest the center of the magnet is different from that on the side of the sensor furthest away from the center of the magnet. More particularly, each sensor element generally has a positive leg and a negative leg each producing an output signal to a comparator or differentiating device, and these legs are adjacent one another in the direction of movement of the object and, thus, are spaced differently with respect to the center of the magnet. Accordingly, the pattern of the magnetic field flux lines relative to the two legs is different, whereby the outputs of the two legs are different. This difference presents problems with respect to obtaining accurate and/or intelligent information with respect to movements of the object. It has been proposed to place a resistor in each leg of each sensor element to compensate for the effect of the difference in the flux field thereacross and, while such an arrangement does compensate for the imbalance, it does so only with respect to a given air gap between the sensor and moving object, and a given temperature in that the sensors are temperature sensitive. Therefore, any change in the air gap and/or temperature introduces error into the readings. A further problem encountered in connection with simply offsetting two sensor elements on laterally opposite sides of the center of the permanent magnet pole face is a considerable decrease in the signal-to-offset error ratio relative to that which exists with a single sensor element coaxial with the magnet axis. In this respect, the offset error level with two offset sensors can be thirty times the offset error level with a single centered sensor, and such a high offset error level makes it extremely difficult to obtain intelligent output signals from the sensors.
As the pair of magnetic sensor elements are inherently fragile, protective casings have been provided wherein the permanent magnet and sensors mounted thereto are placed within a housing and held in position therein by an epoxy material. To provide for protection between the sensor elements and the axially reciprocal rod or rotatable toothed gear, a thin sheet of stainless steel is provided on one side of the housing and between the sensor elements and the moving object. In addition to the physical protection provided the sensor elements by the stainless steel plate, electrical protection against shorts is necessary on the side of the stainless steel plate immediately adjacent the sensor elements. Most often this protection is provided in the form of a thin layer of an insulative material sprayed onto the stainless steel plate. The addition of these protective features, however, necessitate that the magnetic field produced by the permanent magnet must pass through additional layers of material before encountering the object being sensed. Accordingly, the overall sensitivity of the magnetic sensor is reduced.
In addition to the detrimental effects on overall sensor sensitivity, the stainless steel plate is oftentimes destroyed in use through contact between itself and a moving grooved rod or rotatable toothed gear. The vibrational effects encountered in an industrial environment combined with machining inaccuracies in the object being sensed, too often leads to contact between the object being sensed and the stainless steel protective plate, resulting from a scratching or galling of the plate and subsequent failure of the sensing device.
The present invention will be described in detail hereinafter in conjunction with magnetic field sensitive sensor elements in the form of silicon magnetoresistors mounted on a thin ferrite base. This type of sensor element adds an additional problem in connection with the manufacture of permanent magnet type transducers. In this respect, silicon magnetoresistors are extremely fragile, whereby a great deal of care must be exercised in connection with the mounting thereof on the pole face of a permanent magnet. In this respect, the attraction of the magnet during the mounting operation can impact the sensor against the pole face with such force as to shatter the fragile sensor element.